Not applicable.
(1) Field Of The Invention
The present invention generally relates to thermal management of semiconductor devices. More particularly, this invention relates to an electronic assembly that dissipates heat from a flip chip, and also improves the life expectancy of the chip by reducing induced strains in the chip and its solder connections.
(2) Description Of The Related Art
Power flip chips and certain other semiconductor devices require thermal management in order to minimize their operating temperatures. A variety of techniques have been developed for dissipating heat generated by power flip chips. One such technique is disclosed in commonly-assigned U.S. Pat. Nos. 6,180,436 and 6,365,964 to Koors et al., and involves conducting heat from a power flip chip with a heat-conductive pedestal brought into thermal contact with the topside of the chip, i.e., the surface opposite the solder connections that attach the chip to its substrate. A thermally-conductive lubricant is placed between the topside of the chip and the pedestal to fill gaps between the chip and pedestal in order to promote thermal contact, as well as decouple lateral mechanical strains that arise as a result of different thermal expansions and movement between the chip, substrate and pedestal.
While the approach taught by Koors et al. has been successfully implemented, induced stresses can be sufficiently high under severe conditions to fracture the solder connections and even the chip die. Furthermore, thermally-conductive lubricants suitable for placement between a chip and pedestal typically contain a hard particulate filler that can abrade the chip, and the thermal performance of such lubricants is typically compromised as a result of the compositional requirements necessary to achieve adequate lubricity. Finally, thermal energy could be more efficiently dissipated if the contact interface resistance between the chip and pedestal could be reduced.
The present invention is directed to an electronic assembly for conducting heat from a semiconductor device, such as a power flip chip. The assembly is generally constructed to dissipate heat from a chip mounted to a substrate, preferably a laminate such as a printed wiring board (PWB). The substrate has a first region with conductors thereon, a second region surrounding and supporting the first region, and a third region surrounding and supporting the second region, with the second region being more flexible than the first and third regions. The chip is mounted to the first region of the substrate, and has solder connections on a first surface thereof that are registered with the conductors on the first region of the substrate. A heat-conductive member thermally contacts a second surface of the chip oppositely disposed from the first surface. Biasing means contacts the first region of the substrate for biasing the chip into thermal contact with the heat-conductive member.
A significant advantage of the electronic assembly of this invention is that the second region of the substrate improves the mechanical decoupling of strains that arise as a result of different thermal expansions and movement between the chip, substrate, and heat-conductive member, thereby reducing the induced stresses that can cause fracturing of the chip and its solder connections. Induced strains are reduced by the second (xe2x80x9cflexxe2x80x9d) region of the substrate because the chip is located on a separate island of xe2x80x9crigidxe2x80x9d substrate, namely, the first region. The size and mass of the first region can be small, adapted to support a single chip. As such, stresses resulting from chip-to-chip stack-up tolerances can be completely eliminated. Furthermore, the flexibility of the substrate is able to tolerate greater variances in the dimensions of the heat-conductive member, such that fabrication tolerances can be relaxed.
Another advantage of the invention is that, because of the significantly lower induced stresses, a lubricant property is not required between the chip and heat-conductive member. Elimination of this restriction opens up the possibility for using a variety of other materials having better thermal properties than thermally-conductive lubricants. In particular, highly-conductive materials can be used that reduce the tendency for the chip to be abraded during the life of the assembly. Yet another advantage is that a thicker substrate than Koors et al. becomes practical because the chip-to-chip tolerance that would otherwise be taken up by the flexing of a thin laminate substrate can be fully taken up by the flexible second region of the present invention.
Other objects and advantages of this invention will be better appreciated from the following detailed description.